Computer measurements of axis cylinder diameters of radial fibers and "comb" bundle fibers.

In electron micrographs of the squirrel monkey (Saimiri sciureus) brain in the striatal efferents were observed at two different levels in their course: (1) in cross-sectioned radial fiber bundles just before they enter the globus pallidus; (2) in cross-sectioned "comb" bundle fibers just before they enter the substantia nigra. In the radial bundles nearly all of the fibers are myelinated; in the "comb" bundle most are unmyelinated. The polarity of all the "comb" bundle fibers is descending. None of them degenerate following a large lesion in the substantia nigra but they do degenerate following a large lesion in the striatum. Also following this latter lesion the endings with large synaptic vesicles, which make up most of the endings in the globus pallidus and the substantia nigra, degenerate. For computer measurements, electron micrographs of the radial bundle were enlarged photographically to a final magnification of 20,000; those of the "comb" bundle to times 50,000. Measurements of 1309 radial fibers revealed a mean axis-cylinder diameter of 0.68 microns, and measurements of 749 unmyelinated "comb" bundle fibers gave a mean axis-cylinder diameter of 0.21 microns. Myelinated fibers were not included in the "comb" bundle measurements because it contains myelinated fibers of pallidal origin in addition to myelinated fibers of striatal origin. The results here indicate that the striatal efferents undergo a decided decrease in axis-cylinder diameter during their transit through the globus pallidus. It is suggested that the large non-spine bearing neurons in the striatum are the source of the striatal efferents and that they send their axons into the substantia nigra and enroute spend a great quantity of their axoplasm by extending extensive collaterals in both segments of the globus pallidus. This could account for the decreased caliber of the striatal efferents in the "comb" bundle and other findings in the striatum, globus pallidus and substantia nigra.